The present invention relates to a polyethyleneterephthalate (PET) nonwoven fabric having high heat resistance and high strength for a separator for a secondary battery, and to a separator for a secondary battery comprising the same.
Secondary batteries, such as lithium ion secondary batteries, lithium polymer secondary batteries and super capacitors (electric double-layer capacitors and similar capacitors), are required to have high energy density, large capacity and thermal stability depending on the demand trends of high performance, lightness, and large scale for power sources for vehicles.
However, conventional lithium ion secondary batteries using a polyolefin separator and a liquid electrolyte, and conventional lithium ion polymer batteries using a gel polymer electrolyte membrane or a polyolefin separator gel-coated with a polymer electrolyte, have heat resistance inadequate for use as batteries having high energy density and high capacity.
A separator is positioned between the cathode and the anode of a battery to thus be responsible for an insulation function, and maintains an electrolyte to provide an ionic conduction path. Furthermore, when the temperature of the battery is excessively increased, the separator exhibits a shutdown function in such a way that part of the separator is melted to thus close pores in order to block the flow of current. If the separator is melted due to further increased temperature, a large hole is formed, and short-circuit may occur between the cathode and the anode. This temperature is referred to as a short-circuit temperature. Generally, it is preferred that a separator have a low shutdown temperature and a higher short-circuit temperature. In the case of a polyethylene separator, the short-circuit temperature approximates to 140° C. upon overheating of the battery.
With the goal of manufacturing a secondary battery having high energy density and large capacity with a higher short-circuit temperature, a separator is required, which has high heat resistance and thus low thermal shrinkage, and high ionic conductivity and thus superior cycle performance.
To obtain such a separator, US Patent Publication No. 2006/0019154 discloses preparation of a polyolefin separator coated with a porous heat-resistant resin, such as polyimide, polyimide or polyamideimide, having a melting temperature of 180° C. or more.
Japanese Patent Application Publication No. 2005-209570 discloses preparation of a polyolefin separator coated with a heat-resistant resin by coating both surfaces of a polyolefin separator with a heat-resistant resin solution including aromatic polyimide, polyimide, polyethersulfone, polyetherketone or polyetherimide, having a melting temperature of 200° C. or more, and then performing immersion in a coagulant, water washing and drying. As such, a phase separation agent for imparting porosity is added to the heat-resistant resin solution in order to reduce a decrease in ionic conductivity, and the amount of applied heat-resistant resin is limited to 0.5˜6.0 g/m2.
However, immersion in the heat-resistant resin or coating with the heat-resistant resin may close pores of the polyolefin separator, and thus the movement of lithium ions is limited, undesirably deteriorating charge-discharge properties. Therefore, the separator and the electrolyte membrane as disclosed conventionally do not satisfy both heat resistance and ionic conductivity, and the heat-resistant coating may result in deteriorated output properties. Thus, they are difficult to use for batteries having high energy density and large capacity such as batteries for power sources of vehicles, which require superior performance under severe conditions such as rapid charge-discharge, as well as heat resistance.